Definitely +/- 10%.  Would you want to deal with a fabricator whose process
yields 25% variation? (as in .004 +/-.001)  

Dave Schaefer
[log in to unmask]
 
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Dack.Kelly
Sent: Monday, April 26, 2004 11:59 AM
To: [log in to unmask]
Subject: [DC] Etching Plating tolerance specs...

Greetings,

An inconsistency in our fab specs regarding etching/plating tolerance finds
me at a mental "cross-roads" regarding this subject.

The spec for some of our designs express the tolerance in the form of "mils"
--  +/-.001, while other spec express the tolerance in the form of a
percentage -- +/-10%.

In one sense, on a design w/ .010 nominal lines and spacing, these two specs
are the same. However, on a design w/ .005/.005 or .004/.004 lines and
spacing, using a tolerancing method of +/-.001 vs. 10% has drastically
different results.

I would like to see any feedback on the pro's and con's of each method. What
method do you use and why?

-- Kelly

Kelly Dack
PCB Dsnr
IGT (International Game Technology)
Reno, NV

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